Reputation: 1605
I have two functions, each of which return two values (as a list) and which operate over a limited domain:
# for x >= 0
f <- function(x) { return(list(a=x,b=2*x)) }
# for x < 0
g <- function(x) { return(list(a=x/2,b=x/4)) }
I want to do something like this:
fg <- function(x) { return(ifelse(x < 0, g(x), f(x))) }
And I'd like to get this:
> fg(c(1,2))
a$
[1] 1 2
b$
[2] 2 4
> fg(c(-1,-2)
a$
[1] -0.5 -1.0
b$
[2] -0.25 -0.50
Instead, I am getting this:
> fg(c(1,2))
[[1]]
[1] 1 2
[[2]]
[1] 2 4
> fg(c(-1,-2))
[[1]]
[1] -0.5 -1.0
[[2]]
[1] -0.25 -0.50
The "$a" and "$b" labels are getting lost.
With a single input, it's even worse:
> fg(1)
[[1]]
[1] 1
What am I doing wrong?
How can I achieve my objective?
Upvotes: 1
Views: 184
Reputation: 2621
I have modified your code,
# for x >= 0
f <- function(x) { return(list(a=x,b=2*x)) }
# for x < 0
g <- function(x) { return(list(a=x/2,b=x/4)) }
fg <- function(x) {
tmp <- lapply(x, function(y) switch(as.character(y > 0), 'TRUE' = f(y), 'FALSE' = g(y)))
a <- sapply(tmp, function(y) y$a)
b <- sapply(tmp, function(y) y$b)
out <- list(a = a, b = b)
return(out)
}
This is another version, which gives the results you desire but does not need the functions f
and g
. Also, the computation is vectorised:
fg2 <- function(x) {
list(
a = x * (1/2)^(x < 0),
b = x * 2 * (1/8)^(x < 0)
)
}
Below are some examples,
> fg2(1)
$a
[1] 1
$b
[1] 2
> fg2(1:2)
$a
[1] 1 2
$b
[1] 2 4
> fg2(-2:2)
$a
[1] -1.0 -0.5 0.0 1.0 2.0
$b
[1] -0.50 -0.25 0.00 2.00 4.00
Upvotes: 1